AC converters for controlling brushless motors generally comprise six switches. In the event of a defect, each switch can comprise in principle two different characteristics: non-conductive, in other words in the opened switching position, the switch performs a blocking function; or conductive, in other words in the closed switching position, the switch performs a blocking function. A conductively defective switch is also colloquially known as a short circuit. It is then particularly important in applications where safety is concerned to switch the electric motor very quickly into an emergency mode or to switch said electric motor off immediately.
In contrast to mechanical relays, the use of power semiconductor switches is encumbered with the disadvantage that as inductive loads are switched off, depending on the magnitude of the load current and of the inductivity of the load, it is possible to release a quantity of energy of such a magnitude that the power semiconductor switch is destroyed at the moment an inductive load is switched off and as a consequence said power semiconductor switch can no longer fulfill its role. It is necessary to avoid damage to the power semiconductor switches, especially when using power semiconductor switches for brushless motors in applications where safety is critical and where it is necessary to switch off the motor in the power circuit in order to provide a protective function. One application where safety is critical is the use of a brushless electric motor in an electromechanical steering arrangement of a motor vehicle.
For applications of this type where the inductive energy in the load circuit exceeds the admissible absorption capability of a power semiconductor switch, it is known to use instead a mechanical relay that owing to its construction comprises a considerably greater absorption capability in comparison with power semiconductor switches currently available on the market. Although the mechanical relay boosts the switching-off energy which inevitably occurs when switching inductive loads under current, and can therefore be activated at any time, it has very severe availability problems as a mechanical system. A further possibility resides in diverting the amount of energy that arises whilst switching off an inductive load into an energy sink such as by way of example a suppressor diode that relieves the load on the power semiconductor switch.